BACTERIAL GENETICS (LECTURE 11) Flashcards
What are the 2 components that make up the bacterial genome?
There is the bacterial chromosome and the plasmid (non-chromosomal DNA).
Describe the structure of the bacterial chromosome. Where is it located and what genes does it contain?
Double stranded, circular DNA molecule. Found in nucleoid region. Contains essential genes for survival.
Describe the structure of the plasmid (non-chromosomal DNA).
Small, circular, double stranded DNA molecule.
What genes are found in plasmids?
Genes that are not required for survival but that can give bacteria an advantage under changing environmental conditions (such as antibiotic resistance).
What is an episome?
It is a segment of DNA that can exist as a plasmid or be integrated into the bacterial chromosome.
How do bacteria replicate?
Bacteria replicate by binary fission.
Since bacteria replicate by binary fission, what can be said about origins of replication on bacterial chromosomes?
Each chromosome has a single origin of replication.
Do plasmids also replicate during binary fission?
Yes.
What is a possible outcome of genetic recombination in bacteria?
Genetic recombination can produce new bacterial strains.
What is genetic recombination?
Combining DNA from two different sources.
What are the 3 methods of genetic recombination in bacteria? What is the source of external DNA for each?
(1) Transformation (environment), (2) Transduction (bacteriophages) and (3) Conjugation (between bacteria).
Describe bacterial transformation.
Bacterial cells take up foreign DNA from environment (such as a plasmid) and this foreign DNA can remain as a plasmid or be incorporated into the bacterial chromosome by recombination.
Provide an example of bacterial transformation.
Harmless R-cells become transformed to a pathogenic stain when they take up DNA from S-cells.
What is transduction? What does it result in?
Transduction is when genes are transferred form one bacteria to another by a bacteriophage. It results in the random transfer of genes!
Does transduction occur during the lytic or lysogenic life cycle?
Lytic life cycle.
Describe the journey of foreign DNA during transduction.
Pieces of host 1’s DNA are packaged into new bacteriophages during viral assembly. When bacteriophage infects new host, it inserts DNA from the previous host. Recombination occurs between the introduced DNA and the bacterial chromosome.
What is conjugation?
It is the direct transfer of genes between 2 bacterial cells.
Name a bacteria that can perform conjugation.
E. Coli
Describe the steps involved in conjugation.
Donor cell extends an external appendage called the sex pilus. Sex pilus attaches to recipient cell and a cytoplasmic bridge is formed between the 2 cells. DNA can be transferred across the bridge.
What is the F factor?
The ability to form sex pili and to transfer DNA is conferred by a special piece of DNA called the F factor.
What does F factor abbreviate?
Fertility factor.
Is a donor F+ or F-?
A donor has the fertility factor, it is F+.
Is a recipient F+ or F-?
A recipient does not have the fertility factor, it is F-.
Describe F+ cells. (3)
These contain the F factor, F factor is passed onto offspring and it can donate the F factor.
Describe F- cells.
These lack the F factor but can receive the F factor.
Is the F factor in the plasmid or chromosome?
It can be in either one.
Describe conjugation where the F factor is in the plasmid. (3)
The sex pilus forms a cytoplasmic bridge between donor (F+) and recipient (F-) cell. F plasmid replicates and is transferred to F- cell. F- cell becomes F+ cell.
What name is given to F+ cells where the F factor is in the chromosome?
Cells with F factor in chromosome are called high frequency of recombination cells - Hfr cells.
Is a recipient cell guaranteed to become F+ when receiving from an Hfr cell?
No, the recipient cell will often not become F+.
Describe conjugation where the F factor is in the chromosome. (5)
Hfr cell forms bridge with F- cell and transfers the F factor. Some of the bacterial chromosome surrounding the F factor is also transferred. Amount of Hfr DNA transferred is random and depends on when the connection is broken (therefore the recipient will not always become F+). Recombination will occur creating a new bacterial chromosome.
Why are Hfr cells very efficient at creating recombinant bacteria?
Because bacterial genes are delivered along with the F factor during conjugation.
